FMR1 and fragile X syndrome: Fragile X syndrome (FXS) is an X-linked dominant disorder caused by the expansion of a trinucleotide repeat (CGG)n within the first exon of the fragile X mental retardation 1 (FMR1) gene, which silences the expression of the fragile X mental retardation protein (FMRP).1 The absence of FMRP, an important regulator of translation of many messenger RNAs involved in synaptic plasticity,2 leads to substantial intellectual deficits.